<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8" />
    <meta http-equiv="X-UA-Compatible" content="IE=edge" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <title>Document</title>
  </head>
  <body>
    <script>
      let year = +prompt('请输入年份')
      let month = +prompt('请输入月份')
      if (month === 2) {
        if (year % 400 === 0 || (year % 4 === 0 && year !== 0)) {
          alert(`${year}年${month}月有29天`)
        } else {
          alert(`${year}年${month}月有28天`)
        }
      }
      switch (month) {
        case 1:
        case 3:
        case 5:
        case 7:
        case 8:
        case 10:
        case 12:
          alert(`${year}年${month}月有31天`)
          break
        case 4:
        case 6:
        case 9:
        case 11:
          alert(`${year}年${month}月有30天`)
          break
        default:
      }
    </script>
  </body>
</html>
